You can selectively enforce it after the fact- one more charge to stack onto anybody that the .gov does not take a shine to. "Where did you buy your hunting rifle, Joe?" "A gun show, 15 years ago." Joe is innocent until proven guilty, but he'll feel the need to get a Lawyer and go to Court anyway (or fold like a discount tent). This costs Joe money and time. It costs the prosecution money and time, but this is not exactly a fair fight- compared to Joe's resources, the prosecution's are virtually unlimited.

Having a ban on private sales brings more fear and doubt to Joe Sixpack, such that he may opt to sell his guns, or not buy them in the first place. Anything to make ownership more expensive, hazardous and inconvenient will serve to reduce ownership.

Once ownership is reduced to the point that there is no political constituancy that can defend it, then anything goes ....... this is now a Democracy, after all, because the Constitution is a "living document" and means whatever political appointees (judges) say it means.

"How does a bird eat an elephant, you ask? Why, in flocks, one bite at a time." That is the Progressive Model, in a nutshell.

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